Detection and compensation of multiplexer leakage current

ABSTRACT

A multiplexed input/output (I/O) system detects leakage currents on a selected input channel. The system includes a leakage detection multiplexer connected to provide an output selected from one of a plurality of input channels. In addition, the leakage detection multiplexer provides as part of the output measured leakage currents associated with the selected input channel. Based on the detected leakage currents, a determination can made regarding whether the detected leakage currents have compromised the integrity of the multiplexer output. In addition, the detected leakage current can be used to compensate the output provided by the multiplexer to account for the presence of leakage currents on the selected channel.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Divisional Application of U.S. application Ser.No. 12/584,468, filed Sep. 4, 2009, entitled “DETECTION AND COMPENSATIONOF MULTIPLEXER LEAKAGE CURRENT”.

BACKGROUND

The present invention relates to multiplexers, and in particular, to amultiplexer architecture for detecting leakage currents between inputchannels.

A multiplexer is a device that provides an output selected from aplurality of inputs. Multiplexers are beneficial because they allow moreexpensive resources (such as analog-to-digital converters) to be sharedby a plurality of devices. In this way, rather than employ a separateanalog-to-digital converter for each device, a single analog-to-digitalconverter may be employed for a plurality of devices.

A plurality of switches (e.g., metal-oxide semiconductor devices,opto-couplers, etc.) employed by the multiplexer are controlled (i.e.,opened and closed) to select the desired input (i.e., channel) toprovide at the output of the multiplexer. Ideally, the switches whenopen provide total isolation between each of the plurality of channels.In actuality, even when open the switches allow small amounts of currentto leak between channels. This undesirable current is referred to as‘leakage current’, the result of which can have a negative impact on theintegrity of the signal provided at the output of the multiplexer.

In many applications, leakage currents are insubstantial and do notsignificantly affect the integrity of the multiplexer output. However,factors such as temperature and device-to-device potentials (i.e., inputdevices of the multiplexer being maintained, either intentionally orunintentionally at significantly different potentials), either alone orin combination, increase the leakage current associated with switchesemployed by the multiplexer.

In many applications (such as process control applications), even asmall change in the integrity of the signal provided by the multiplexermay result in significant application errors. It would therefore bebeneficial to detect the presence of leakage currents and/or compensatethe output signal provided by the multiplexer in response to detectedleakage currents.

SUMMARY

A multiplexed input/output (I/O) system detects leakage currents on aselected input channel. The system includes a leakage detectionmultiplexer connected to provide an output selected from one of aplurality of input channels and a leakage output indicative of leakagecurrent associated with a selected input channel. In response to theleakage output exceeding a threshold value, a controller providesnotification indicating a loss of integrity associated with the selectedinput channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an input/output (I/O) system for providingleakage current detection/compensation according to an embodiment of thepresent invention.

FIGS. 2A and 2B are circuit diagrams illustrating in more detailembodiments of the leakage current detection multiplexer according tothe present invention.

FIGS. 3A and 3B are graphs illustrating the correlation between leakagecurrent and the integrity of measurements provided by the multiplexerand limits used to indicate excessive leakage current.

FIG. 4 is a circuit diagram illustrating in more detail the leakagecurrent detection multiplexer according to an embodiment of the presentinvention.

FIG. 5 is a circuit diagram illustrating in more detail the leakagecurrent detection multiplexer according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

In general, the present invention is a multiplexed input/output systemthat provides for the detection and/or compensation of leakage currentsassociated with the multiplexer. In particular, the multiplexer employedby the present invention includes with respect to each input channel atleast one leakage current sense resistor, as well as additional buslines and switches necessary to measure the voltage across the leakagecurrent sense resistor. Based on the measured voltage across the leakagecurrent sense resistor, the leakage current associated with a particularchannel can be estimated and employed to determine whether the integrityof the output provided by the multiplexer has been compromised by thedetected leakage current. A warning signal can be provided in responseindicating the potential loss of integrity associated with theparticular channel and/or channels.

In addition to detecting the presence of leakage currents, the presentinvention may provide compensation to the output signal provided by themultiplexer based on the detected leakage currents. The compensationcorrects for errors introduced into the output signal by the leakagecurrent. A benefit of this approach is signal integrity is maintaineddespite conditions (e.g., high temperatures, large device-to-devicepotentials) that would render a typical multiplexer inoperable.

FIG. 1 is a block diagram of input/output (I/O) system 10 according toan embodiment of the present invention. System 10 includes leakagecurrent detection multiplexer 12 (hereinafter multiplexer 12),analog-to-digital (A/D) converter 14, and controller 16. Multiplexer 12is connected to a plurality of input channels Ch_1, Ch_2 . . . Ch_3(collectively, “input channels”), each connected to one of plurality ofsensor devices 18 a, 18 b, and 18 c, respectively (collectively, “sensordevices 18”). Each input channel may consist of one or more input lines(i.e., terminals). For example, sensor device 18 a may be a two-leadthermocouple. As such, multiplexer 12 would include two input terminalswith respect to sensor device 18 a. Other types of sensor devices employvarious lead configurations. As such, multiplexers are typicallydesigned to accommodate a variety of sensor types, commonly employingfour input terminals with respect to each channel.

Multiplexer 12 includes a plurality of switches (embodiments of whichare described in more detail with respect to FIGS. 2A, 2B, 4 and 5) thatare selectively controlled (i.e., opened and closed) by controller 16 todetermine which of the plurality of input channels to provide at thecommon output of multiplexer 12. To select a particular input channel(e.g., input channel Ch_1), switches associated with the selectedchannel are closed and the input provided at the selected input channelis provided at a common output of multiplexer 12. Switches connected tothe remaining, unselected input channels (e.g., input channels Ch_2,Ch_3) remain open to isolate the non-selected input channels from theselected input channel.

The presence of a potential difference (labeled ‘Vs−s’) between adjacentsensors (e.g., between sensors 18 a and 18 b) can contribute to thepresence of crosstalk or leakage current between the respective inputchannels. Leakage currents distort the input signal provided at aselected input channel, and if significant will result in the erroneousinterpretation of the process variable measured by a correspondingsensor. As described in more detail below, the architecture ofmultiplexer 12 according to embodiments of the present invention allowleakage currents associated with one or more of the plurality of inputchannels to be detected. The detected leakage current (labeled‘leak_det’) is provided as an output of multiplexer 12 along with thesensor input provided by the selected input channel (labeled ‘Sel_Ch’)to A/D converter 14. A digital representation of the detected leakagecurrent is provided to controller 16 to determine whether an alarm ornotification should be sent indicating the loss of integrity associatedwith the selected input channel and/or to allow controller 16 to providecompensation to the signal provided by the selected input channel basedon the detected leakage current.

FIGS. 2A and 2B are circuit diagrams illustrating in more detailembodiments of the leakage current detection multiplexer (labeledmultiplexer 22 a and multiplexer 22 b, respectively) according to thepresent invention. In the embodiment shown in FIG. 2A, multiplexer 22 ais connected to receive sensor data from a plurality of input channels,two of which are shown (i.e., input channels Ch_1 and Ch_2). Multiplexer22 a includes a plurality of input terminals Ch_1 a, Ch_1 c, Ch_2 a, andCh_2 c, a plurality of switches S_1 a, S_1 b, S_1 c, S_2 a, S_2 b, andS_2 c, a plurality of leakage current sense resistors R_1 b and R_2 b,and a plurality of output terminals Sel_Ch_a, Sel_Ch_b, and Sel_Ch_cthat make up the common output channel of multiplexer 22 a.

Components employed by multiplexer 22 a are labeled to indicate both theinput channel and output terminal associated with the component. Forexample, the input terminal associated with input channel Ch_1,connected to provide an output at output terminal Sel_Ch_a (via switchS_1 a) is labeled Ch_1 a. The other input terminal associated withchannel Ch_1, connected to provide an output at output terminal Sel_Ch_c(via switch S_1 c) is labeled Ch_1 c. The leakage sense resistorassociated with input channel Ch_1 is labeled R_1 b because it isconnected (via switch S_1 b) to output terminal Sel_Ch_b. Componentsassociated with input channel Ch_2 are similarly labeled, with inputterminals Ch_2 a and Ch_2 c, switches S_2 a, S_2 b, and S_2 c, andleakage current resistor R_2 b. Input terminals Ch_2 a and Ch_2 c areconnected to output terminals Sel_Ch_a and Sel_Ch_c, respectively,through switches S_2 a and S_2 c, respectively. Leakage sense resistorR_2 b is connected to output terminal Sel_Ch_b via switch S_2 b.

To facilitate the description of multiplexer 22 a, sensors 18 areillustrated here as simple two-terminal thermocouple devices, eachdevice including a high-lead resistance Rh and a low-lead resistance R1connected to respective input terminal (e.g., sensor device 18 a isconnected to input terminals Ch_1 a and Ch_1 c). A typical thermocouplewould further include an artificial cold junction using some otherthermally sensitive device, such as a thermistor or diode, to measurethe temperature of the input connections at the instrument although forpurposes of this discussion the simple configuration illustrated willsuffice.

To select a particular input channel to provide at the common outputchannel of multiplexer 22 a, each switch associated with the selectedinput channel is closed. As shown in FIG. 2A, switches S_1 a, S_1 b andS_1 c are closed to select input channel Ch_1. Conversely, switchesassociated with non-selected channels are opened to isolate the selectedchannel from the non-selected channels. Ideally, an open switchrepresents an infinite resistance that effectively prevents any currentfrom flowing through a non-selected channel. From a practicalstandpoint, however, an open switch has a finite resistance that variesbased on individual characteristics of the switch as well as externalfactors such as the temperature associated with the switch. Coupled withthe presence of sensor-to-sensor potential differences (illustrated bythe voltage ‘Vs−s’), leakage currents may be induced through the openswitches as shown in FIG. 2A, in which leakage currents ia, ib, and icflow through open switches S2-A, S2-B and S2-C, respectively. Becauseeach of the channels is connected to a common output (output terminalsSel_Ch_a, Sel_Ch-b, and Sel_Ch_c), the leakage currents ia, ib and icflow through open switches S_2 a, S_2 b and S_2 c and are providedthrough closed switches S_1 a, S_1 b and S_1 c, respectively.

Resulting leakage currents ia, ib, and ic flow through thermocoupleleads Rh_1 and R1_1, resulting in a voltage drop that modifies (assumingunequal resistances associated with leads Rh_1 and R1_1) the potentialdifference provided by the thermocouple at input terminals Ch_1 a andCh_1 c. As a result, the output provided by multiplexer 22 a (at outputterminals Sel-Ch_a and Sel_Ch_c) representing the sensed temperaturewill include some distortion due to the leakage current.

Detection of leakage current by multiplexer 22 a is provided by theaddition of leakage sense resistor (e.g., leakage sense resistor R_1 bprovided with respect to input channel Ch_1) and an additional bus lineand output terminal (e.g., output terminal Sel_Ch_b) for measuring thevoltage drop across leakage sense resistor R_1 b. To determine theleakage current associated with a particular channel, the plurality ofswitches associated with the selected channel are closed, including theswitch associated with the leakage sense resistor, and the voltagesassociated with each bus line are provided at the respective outputterminals of multiplexer 22 a.

For example, to measure leakage currents associated with channel Ch_1,switches S_1 a, S_1 b, and S_1 c are closed (all remaining switchesincluded within multiplexer 22 remain open), and the voltage acrosscurrent sense resistor R_1 b is measured by way of the voltagedifference provided at output terminals Sel_Ch_b and Sel_Ch_c. Thevoltage difference across current sense resistor R_1 b, combined withknowledge of the resistance value of resistor R_1 b, allows the leakagecurrent ib to be determined. In this embodiment, the current intoterminal Ch_1 a is not measured, and the current into terminal Ch_1 c isknown only to the extent that the leakage current ib can be attributedto leakage current is (the sum of which determines the actual leakagecurrent into terminal Ch_1 c). However, for purposes of detecting a lossof sensor integrity (i.e., loss of integrity associated with the sensordata provided by the selected input channel to the common outputchannel, in this case, at output terminals Sel_Ch_a and Sel_Ch_c),knowledge regarding the magnitude of the leakage current through one ofthe bus lines is typically sufficient.

The voltage values provided at output terminals Sel_Ch_a, Sel_Ch_b andSel_Ch_c are provided to A/D converter 14, which converts the analogvoltage values to digital values. A/D converter 14 provides the digitalvalues to controller 16, which interprets the provided values. Forexample, a two-terminal thermocouple device provides a voltagedifference at the terminals that reflects the measured temperature.Controller 16 would interpret the voltages provided at output terminalsSel_Ch_a and Sel_Ch_c (connected to input terminals Ch_1 a and Ch_1 c,respectively) to determine the temperature measured by sensor device 18a. In addition, controller 16 determines based on the detected leakagecurrent (represented by the voltage difference provided between outputterminals Sel_Ch_b and Sel_Ch_c) whether the detected leakage currenthas compromised the integrity of the output provided by multiplexer 22a. This may include comparing the measured leakage current to athreshold value or reporting the sensed leakage current to a user orcontrol room. In other embodiments, controller 16 may compensate thesignal provided by the sensor to account for the leakage current, butaccurate compensation is improved by knowledge of the leakage currentthrough each input terminal of the selected channel (as described withrespect to FIGS. 4 and 5).

Similar operations may be performed with respect to each input channel.Depending on the application, A/D converter 14 and controller 16, bothof which represent relatively expensive operations, may onlyperiodically review the leakage current associated with each of theplurality of inputs. This is typically acceptable as the factors givingrise to large leakage currents (e.g., temperature and sensor-to-sensorpotential) are typically slow to change. In other applications, however,it may be beneficial to continuously monitor the presence of leakagecurrents.

FIG. 2B is a circuit diagram illustrating in more detail anotherembodiment of the leakage current detection multiplexer (labeledmultiplexer 22 b to differentiate between multiplexer 22 a describedwith respect to FIG. 2A). Multiplexer 22 b includes the same componentsdescribed with respect to multiplexer 12 a shown in FIG. 2A, including aplurality of input terminals Ch_1 a, Ch_1 c, Ch_2 a and Ch_2 c, aplurality of switches S_1 a, S_1 b, S_1 c′, S_2 a, S_2 b and S_2 c′, aplurality of leakage current sense resistors R_1 b and R_2 b, and aplurality of output terminals Sel_Ch_a, Sel_Ch_b and Sel_Ch_c.

The difference between this embodiment and that shown in FIG. 2A is theconnection of switches S_1 c′ and S_2 c′ (the prime notation indicatesschematic differences between the embodiments). In FIG. 2A, a first sideof switch S_1 c was connected to input terminal Ch_1 c and the secondside was connected to output terminal Sel_Ch_c. In this embodiment, thefirst side of switch S_1 c′ is connected between switch S_1 b andcurrent leakage resistor R_1 b. The second side of switch S_1 c′ remainsconnected to output terminal Sel_Ch_c. Likewise, the first side ofswitch S_2 c′ is connected between switch S_2 b and currently leakageresistor R_2 b.

The benefit of this approach is a majority of the leakage current thatflows into terminal Ch_1 c is attributable to leakage current ib (asopposed to a combination of leakage currents ib and ic), which can bemeasured based on the measured voltage drop across current leakageresistor R_1 b. Assuming leakage current is due, in large part, to thepresence of a large sensor-to-sensor potential difference Vs−s, byconnecting the first side of switch S_2 c′ on the opposite side ofswitch S_2 b, a majority of the potential difference will drop acrossopen switch S_2 b (due to the relatively high, although not infinite,resistance associated with an open switch). As a result, the first sideof switch S_2 c′ is connected to a node of significantly lower potentialthan the first side of switch S_2 c (as shown in FIG. 2A), and themagnitude of leakage current is flowing through open switch S_2 c′ issignificantly less than the leakage current ib flowing through openswitch S_2 b.

As a result, the leakage current estimated based on the voltagedifference measured across leakage sense resistor R_1 b more accuratelyportrays the leakage current flowing into input terminal Ch_1 c. Theleakage current provided to input terminal Ch_1 a remains unknown inthis embodiment, but once again may be estimated based on the measuredleakage current associated with input terminal Ch_1 c.

FIGS. 3A and 3B are graphs illustrating the correlation betweensensor-to-sensor voltage and temperature with leakage current magnitudesas well as an example of how leakage current thresholds are employed todetermine loss of sensor integrity. FIGS. 3A and 3B are from prototypesbased on the circuit diagram described with respect to FIG. 2A, but theprinciples remain valid for the embodiment described with respect toFIG. 2B.

FIG. 3A illustrates how sensor-to-sensor potential and temperatureaffects leakage current. The y-axis represents current magnitudes (e.g.,nano-Ampere (ηA) units) and the x-axis represents ambient temperatures(e.g., Celsius units) associated with a multiplexer (i.e., thetemperature does not necessarily represent temperature value sensed bythe sensor, only ambient temperatures associated with the multiplexer).Each line represents leakage currents sensed with respect to varyingmagnitudes of sensor-to-sensor potential Vs−s, labeled ‘−600 V’, ‘−100V’, ‘0 V’, ‘50 V’, ‘100 V’, and ‘600 V’, respectively. Threshold limitsidentifying the leakage current amplitude at which point the integrityof sensed process variables (pv) is compromised are illustrated at 30 ηAand at −30 ηA (labeled ‘leakage limit’). These thresholds are determinedbased at the point (shown in FIG. 3B) at which the sensed processvariable exceeds defined specification limits (provided at 15 μV and −15μV).

As shown in FIG. 3A, leakage currents increase with increasingtemperature and with increased sensor-to-sensor potential Vs−s. Inparticular, when the sensor-to-sensor potential Vs−s equals 600 V(represented by the line labeled ‘600 V’), the leakage current matchesthe leakage limit when the ambient temperature reaches 75° C., andexceeds the leakage limit when the ambient temperature reaches 85° C.Exceeding the leakage limit indicates that errors associated with thesensed process variables have exceeded specification limits (shown inFIG. 3B) such that the integrity of the sensor signal has beencompromised.

FIG. 3B illustrates how leakage currents generated by increasingsensor-to-sensor potential Vs−s and increasing temperature negativelyimpacts the sensed process voltage, and therefore the integrity of thesensed process variable. The y-axis represents error values (micro-Volts(μV) units) introduced into the sensed process variable and the x-axisrepresents ambient temperatures (Celsius) associated with a multiplexerwith the same scale as that employed with respect to FIG. 3A. Onceagain, each line represents process variable errors measured withrespect to varying amplitudes of sensor-to-sensor potential Vs−s,labeled ‘−600 V’, ‘−100 V’, ‘−50 V’, ‘0 V’, ‘50 V’, ‘100 V’, and ‘600V’, respectively. Specification limits (labeled ‘pv limits’) are definedto indicate the point at which the integrity of sensed process variableshas been compromised (e.g., 15 μV and −15 μV). In this simulation,errors in the sensed process variable exceed the specification limit ata temperature of 75° C. and a sensor-to-sensor voltage of |600| V (i.e.,at 600 V and −600 V). The leakage current generated at this temperatureand sensor-to-sensor potential, as shown in FIG. 3A, becomes the leakagecurrent threshold (also shown in FIG. 3A). In this way, leakage currentsthat exceed the defined threshold indicate a loss of integrity, asdefined by the sensed process voltage.

Leakage limits are determined with respect to a particular sensor basedon knowledge regarding how leakage currents will affect sensorintegrity. The leakage current limits can be estimated based oninformation known about the type of sensor being employed, including theresistance of the sensor and the differential resistance associated witheach lead, or may be determined by applying a known current to thesensor and measuring a resulting voltage (in much the same way a currentis applied to a two-lead resistive temperature device (RTD)) todetermine the resistance of a particular sensor. Knowing the resistanceof the sensor (e.g., with respect to thermocouple devices, knowing theresistivity ratio associated with each lead of the thermocouple)determines the effect leakage currents will have on the integrity of thesignal. For example, with respect to thermocouples, the larger theresistivity ratio between terminals, the more prominent the effect theleakage current will have on sensor integrity.

FIG. 4 is a circuit diagram illustrating another embodiment of leakagecurrent detection multiplexer (labeled here as multiplexer 24) accordingto the present invention.

In the embodiment shown in FIG. 4, multiplexer 24 is once againconnected to receive sensor data from a plurality of input channels, twoof which are shown (input channels Ch_1 and Ch_2). Once again, thedescription is simplified by the assumption of two-terminal devicesconnected to each of the respective input channels. Multiplexer 24includes a plurality of input terminals Ch_1 a, Ch_1 d, Ch_2 a and Ch_2d, a plurality of switches S_1 a, S_1 b, S_1 c, S_1 d, S_2 a, S_2 b, S_2c and S_2 d, a plurality of leakage sense resistors R_1 b, R_1 c, R_2 band R_2 c, and a plurality of output terminals Sel_Ch_a, Sel_Ch_b,Sel_Ch_c and Sel_Ch_d making up the common output channel of multiplexer24.

Components employed by multiplexer 24 are once again labeled to indicateboth the input channel and output terminal associated with thecomponent. For example, the input terminal associated with input channelCh_1, connected to provide an output at output terminal Sel_Ch_a (viaswitch S_1 a), is labeled Ch_1 a. The other input terminal associatedwith channel Ch_1, connected to provide an output at output terminalSel_Ch_d (via switch S_1 d), is labeled Ch_1 d. The leakage senseresistors associated with input channel Ch_1 connected (via switches S_1b and S_1 c, respectively) to output terminals Sel_Ch_b and Sel_Ch_c arelabeled R_1 b and R_1 c, respectively.

In contrast with the embodiment described with respect to FIGS. 2A and2B, this embodiment includes a leakage sense resistor associated witheach of the plurality of input terminals. Although this embodimentresults in an increase in the complexity associated with multiplexer 24,a benefit of the additional bus lines and associated leakage senseresistors is the ability to know the leakage current at each inputterminal. Knowledge of the leakage current through each input terminalcan be used to compensate the sensor signal provided on a selected inputchannel.

For instance, in the example shown in FIG. 4, switches S_1 a, S_1 b, S_1c, and S_1 d are closed to select input channel Ch_1. Switchesassociated with non-selected channel remain open. The sensor inputprovided by sensor 18 a is provided as a voltage difference at outputterminals Sel_Ch_a and Sel_Ch_d, respectively. Leakage current ibflowing into input terminal Ch_1 a is determined based on the voltagedrop across leakage sense resistor R_1 b, measured as the voltagedifference between output terminals Sel_Ch_a and Sel_Ch_b. Leakagecurrent is flowing into input terminal Ch_1 d is determined based on thevoltage drop across leakage sense resistor R_1 c, measured as thevoltage difference between output terminals Sel_Ch_c and Sel_Ch_d. Inthis way, the embodiment shown in FIG. 4 provides information regardingthe leakage current provided to both input terminals (e.g., inputterminal Ch_1 a and Ch_1 d) associated with a particular channel.

The present embodiment does not measure the leakage currents ia and id,and thus the actual leakage current provided to the input terminals Ch_1a and Ch_1 d is known only to the extent the measured leakage currentsib and ic can be attributed to the remaining bus lines. Applicants inwhich three- or four-terminal sensor devices are connected to aparticular input channel may required knowledge of the leakage currentsthrough those additional bus lines (as shown in FIG. 5). However, amajority of the leakage current that flows into terminals Ch_1 a andCh_1 d is attributable to leakage current ib and ic, respectively, asopposed to a combination of leakage currents ia and ib, and leakagecurrents ic and id. Leakage currents ib and ic are known based on themeasured voltage drop across leakage resistor R_1 b and R_1 c,respectively. This is attributable, once again, to the large potentialacross switches S_2 b and S_2 c, respectively, and the relatively smallpotential across switches S_2 a and S_2 d, which results in relativelysmall leakage currents ia and id.

As a result, the leakage current estimated based on the voltagedifference measured across leakage sense resistors R_1 b and R_1 caccurately portrays the leakage current flowing into input terminalsCh_1 a and Ch_1 d. Calculating the effect of the leakage current on thevoltage between the input terminals Ch_1 a and Ch_1 d requires knowledgeof the high-side resistance (Rh_1) and low-side resistance (R1_1). Inone embodiment, a regular 2-wire ohmic measurement across thethermocouple, measuring a total resistance associated with the device,coupled with knowledge of the type of device, which determines the ratiobetween the high-side resistance and the low-side resistance, allows thehigh-side resistance and low-side resistance to be individuallydetermined. The compensation to be added to a thermocouple device havinga known high-side resistance and low-side resistance is defined by thefollowing exemplary equation:Vcomp=−ib*Rh _(—)1+ic*Rl _(—)1  Eq. 1

The voltage compensation calculated using Equation 1 takes into accountthe voltage drop attributable to leakage current ib flowing throughhigh-side resistor Rh_1 and the voltage drop attributable to leakagecurrent ic flowing through low-side resistor R1_1.

For purposes of detecting a loss of sensor integrity due to the presenceof leakage currents, knowledge regarding the leakage currents at one ofthe input terminals is typically sufficient. With information regardingthe flow of leakage currents into each terminal of the input channel,along with knowledge regarding the high-side resistance Rh_1 andlow-side resistance R1_1, the effect of the leakage current on thevoltage provided at input terminals Ch_1 a and Ch_1 d can becompensated.

The outputs provided by multiplexer 24, including outputs representingthe sensor signal (e.g., voltage difference between output terminalsSel_Ch_a and Sel_Ch_d) and outputs representing the voltage drop acrossthe respective leakage sense resistors (e.g., voltage difference betweenoutput terminals Sel_Ch_a and Sel_Ch_b, and between output terminalsSel_Ch_c and Sel_Ch_d) are converted to digital values by A/D converter14 and provided to controller 16. Based on the provided outputs,controller 16 calculates the process variable measured by the sensorconnected to the selected channel (in this case, the temperaturemeasured by the thermocouple) and compensates the measured sensor signalbased on the calculated leakage currents (e.g., as shown in Equation 1).A benefit of this approach is despite the presence of leakage currentsthat would otherwise compromise the integrity of the sensor signal, thepresent invention is able to provide compensation to correct for thedetected presence of leakage currents.

Similar operations may be performed with respect to each input channel.Depending on the application, A/D converter 14 and controller 16, bothof which represent relatively expensive operations, may onlyperiodically review the leakage current associated with each of theplurality of inputs, applying the previously calculated compensationsignal to the sensor signal. This is typically acceptable as the factorsgiving rise to large leakage currents (e.g., temperature andsensor-to-sensor potential) are typically slow to change. In otherapplications, however, it may be beneficial to monitor the leakagecurrents with respect to each selected channel. This improves theaccuracy of the signal by continually updating the compensation providedto the sensor signal.

FIG. 5 is a circuit diagram illustrating in more detail anotherembodiment of the leakage current detection multiplexer (labeled here asmultiplexer 26). As compared with the multiplexer described with respectto FIG. 4, multiplexer 26 includes additional bus lines associated witheach channel and additional leakage sense resistors for measuring theleakage resistor into the input terminals of multiplexer 26. Multiplexer26 includes a plurality of input terminals Ch_1 b, Ch_1 f, Ch_2 b, andCh_2 f, a plurality of switches S_1 a, S_1 b, S_1 c, S_1 d, S_1 e, S_1f, S_2 a, S_2 b, S_2 c, S_2 d, S_2 e, and S_2 f, a plurality of leakagesense resistors R_1 a, R_1 c, R_1 d, R_1 e, R_2 a, R_2 c, R_2 d, and R_2e, and a plurality of output terminals Sel_Ch_a, Sel_Ch_b, Sel_Ch_c,Sel_Ch_d, Sel_Ch_e, and Sel_Ch_f making up the common output channel.Components are once again labeled to indicate both the input channel andoutput terminal associated with the component.

The additional bus lines allow multiplexer 26 to provide leakage currentdetection associated with sensors employing four terminals per channel.For instance, although sensor 18 a is illustrated here as a thermocoupledevice, it may be replaced with a resistive temperature device (RTD)having a resistance that varies with temperature. RTD devices may bethree-terminal devices or four-terminal devices. In a four-terminaldevice, current is provided to the RTD device via two terminals. Theother two terminals are high-impedance paths for measuring the voltagegenerated across the RTD in response to the provided current. With aknown current, the measured voltage is used to determine the resistanceof the RTD, and therefore the corresponding temperature of the device.

In this embodiment, only leakage currents provided at each inputterminal are labeled, as opposed to previous embodiments in whichleakage currents were associated with each bus line. Therefore, leakagecurrents i1 and i2 are provided between terminals Ch_2 b and Ch_1 b, andleakage currents i3 and i4 are provided between terminals Ch_2 f andCh_1 f.

To select a particular input channel to provide at the common outputchannel of multiplexer 26, each switch associated with the selectedinput channel is closed. In the example shown in FIG. 5, switches S_1 a,S_1 b, S_1 c, S_1 d, S_1 e, and S_1 f are closed to select channel Ch_1.Conversely, switches associated with non-selected channels are opened(or remain open) to isolate the selected channel from the non-selectedchannels. As discussed above, leakage currents may develop between theselected channel and the non-selected channel. In this embodiment,leakage currents may develop between each of the plurality of bus lines,but the leakage current is typically insubstantial in bus linesassociated with switches S_1 b and S_1 f. For example, switch S_2 f isconnected between switch S_2 e and resistor R_2 e. When switch S_2 e isopen (i.e., high impedance), a majority of the sensor-to-sensor voltageVs−s is provided across switch S_2 e, such that switch S_2 f has aminimal voltage drop. As a result, minimal leakage current is providedvia switch S_2 f to switch S_1 f. The same is true with respect toswitch S_2 b, with little or no leakage current being provided to switchS_1 b.

With this in mind, leakage currents i1, i2, i3 and i4 can be accuratelydetermined by measuring the voltage across leakage sense resistor R_1 a,R_1 c, R_1 d, and R_1 e, respectively. Leakage current i1 is determinedby measuring the voltage on either side of current sense resistor R_1 avia output terminals Ch_Sel_a and Ch_Sel_b. The measured voltage acrossleakage sense resistor R_1 a, combined with knowledge of the resistancevalue of resistor R_1 a allows the leakage current i1 to be determined.Likewise, the leakage current i2 is determined by measuring the voltageon either side of current sense resistor R_1 c via output terminalsCh_Sel_b and Ch_Sel_c. Resistors R_1 d and R_1 e are similarly employedto measure the leakage currents i3 and i4 flowing into input terminalCh_1 f. Leakage currents i1 and i2 are combined to determine the leakagecurrent into input terminal Ch_1 b and leakage currents i3 and i4 arecombined to determine the leakage current into input terminal Ch_1 f.

As discussed above, the voltage values provided at the output terminalsSel_Ch_a, Sel_Ch_b, Sel_Ch_c, Sel_Ch_d, Sel_Ch_e, and Sel_Ch_f areprovided to A/D converter 14, converted to digital values, and providedto a controller (e.g., controller 16 shown in FIG. 1) for analysis. Forinstance, sensor values are based on the voltage difference providedbetween output terminals Sel_Ch_b and Sel_Ch_f. Leakage current i1 isbased on the voltage difference provided between output terminalsSel_Ch_a and Sel_Ch_b and leakage currents i2, i3 and i4 are based onthe respective voltage differences sensed across leakage sense resistorsR_1 c, R_1 d and R_1 e.

Based on the provided output, controller 16 calculates the processvariable measured by the sensor connected to the selected channel (e.g.,the temperature sensed by sensor device 18 a) and determines whether themagnitude of the leakage currents has compromised the integrity of themultiplexer output. Knowledge of the magnitude of leakage currents intoeach terminal is beneficial, as it allows controller 16 to betterdetermine the effect the leakage currents will have on the sensedprocess variable.

In addition, knowledge of the leakage currents flowing into both inputterminals allows the controller to calculate a compensation signal(e.g., voltage) that is added to the output provided by the selectedsensor in order to compensate for the effects of the leakage currentprovided to respective leads of sensor device 18 a. The additional buslines allow leakage currents to be measured with respect to inputchannels that include multiple terminals (e.g., input channels connectedto RTD devices). A benefit of this approach is despite the presence ofleakage currents that would otherwise compromise the integrity of thesensor signal, the present invention is able to provide compensation tocorrect for the detected presence of leakage currents. In an exemplaryembodiment, controller 16 employs the following equation to calculatethe compensation to be applied to the sensor output provided by athermocouple with lead resistances Rh_1 and R1_1. As discussed withrespect to Eq. 1, knowledge of the high-side resistance Rh_1 value andlow-side resistance R1_1 value, in the case of a thermocouple device, isrequired for the compensation calculation.Vcomp=−((i1+i2)*Rh _(—)1+i2*R _(—)1c)+((i3+i4)*Rl _(—)1+i3*R_(—)1d))  Eq. 2

The voltage compensation calculated as shown in Equation 2 takes intoaccount the effect of leakage currents on the voltage drop across thehigh-side resistor attributable to leakage currents i1 and i2 and thevoltage drop across sense resistor R_1 c attributable to leakage currenti2, as well as the voltage drop across the low-side resistorattributable to leakage currents i3 and i4 and the voltage drop acrosssense resistor R_1 d attributable to leakage current i3.

Similar operations may be performed with respect to each input channel.Depending on the application, A/D converter 14 and controller 16, bothof which represent relatively expensive operations, may onlyperiodically review the leakage current associated with each of theplurality of inputs, applying the previously calculated compensationsignal to the sensor signal. This is typically acceptable as the factorsgiving rise to large leakage currents change relatively slowly. In otherapplications it may be beneficial to monitor the leakage currents withrespect to each selected channel. This improves the accuracy of thesignal by continually updating the compensation provided to the sensorsignal.

Although the present invention has been described with reference toparticular embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the scopeof the invention as claimed. For example, the multiplexer of the presentinvention may be configured to communicate with devices other than thesimple two-terminal thermocouples described with respect to FIGS. 2A,2B, 4 and 5. Some devices require additional input terminals withrespect to each input channel. As a result, the multiplexer wouldinclude additional bus lines connected to each of the input terminalsand additional output terminals to communicate each of inputs associatedwith a particular channel. In order to measure leakage currentsassociated with one or more the terminals, additional leakage senseresistors would be required with respect to one or more the inputterminals. Additional bus lines would also be required in order tomeasure the voltage drop across each of the provided leakage senseresistors.

In addition, each of the embodiments describes the multiplexer, the A/Dconverter and the controller as individual components, but workersskilled in the art will recognize that these components could beincorporated together as a single element. For instance, the functionsperformed by each of these components may be implemented with the use ofan application specific integrated circuit (ASIC) or similar device.

The invention claimed is:
 1. A multiplexed input/output (I/O) systemincluding: a leakage detection multiplexer for receiving a sensor inputat each of a plurality of input channels, for providing a sensor outputselected from one of the plurality of input channels at an outputchannel associated with the selected input channel, and for providing aleakage output indicative of leakage current associated with a selectedinput channel at the output channel associated with the selected inputchannel, wherein the leakage detection multiplexer includes at least oneleakage sense resistor connected to each of plurality of input channels,wherein the leakage output represents the voltage across the leakagesense resistor associated with the selected input channel; and acontroller for generating a notification when the output indicative ofleakage current exceeds a threshold value indicating a loss of integrityassociated with the selected input channel.
 2. The multiplexed I/Osystem of claim 1, wherein the leakage detection multiplexer includes aplurality of input terminals associated with each of the plurality ofinput channels, wherein the leakage output is indicative of a leakagecurrent associated with at least one of the plurality of inputterminals.
 3. The multiplexed I/O system of claim 1, wherein the leakagedetection multiplexer includes a plurality of input terminals associatedwith each of the plurality of input channels, wherein the leakage outputis indicative of leakage currents associated with each of the pluralityof input terminals associated with a particular input channel.
 4. Themultiplexed I/O system of claim 3, wherein the controller calculates acompensation signal based on the leakage output provided with respect toeach of the plurality of input terminals and adds the compensationsignal to the sensor output to generator a compensated sensor outputthat accounts for the detected presence of leakage currents.
 5. Themultiplexed I/O system of claim 4, wherein the controller calculates thecompensation signal based, in addition, on information regarding a typeof sensor connected to the selected input channel and resistive valuesassociated with the sensor connected to the selected input channel todetermine the effect the monitored leakage current has on the sensoroutput provided by the multiplexer.